1. Field of the Invention
The present invention relates to an electrostatic ink-jet recording apparatus, and more particularly to an electrostatic ink-jet recording apparatus for recording data by using an ink obtained by dispersing toner particles in a carrier solution and thereby ejecting the toner particles in accordance with an electrostatic force.
2. Description of the Prior Art
As described in PCT Publication Number WO93/11866, a conventional electrostatic ink-jet recording apparatus comprises an electrostatic ink-jet recording head and a counter electrode set at the back of a recording paper to form an electric field between the counter electrode and the ink-jet recording head. The ink-jet recording head has an ink chamber for temporarily storing an ink solution supplied from an ink tank or the like. An ejection electrode is formed at an end of the ink chamber and driven to eject the ink. The front end of the ejection electrode faces the counter electrode. The ink solution in the ink chamber is supplied up to the front end of the ejection electrode due to the surface tension of the ink solution and thereby, ink meniscuses are formed at the front end of the ejection electrode.
The ink solution used for the ink-jet recording head contains electrified toner particles for producing a color. The electrified toner particles are electrified into the positive polarity due to a zeta potential. However, while no voltage is applied to the ejection electrode, the ink solution is electrically kept neutral. The polarity of the zeta potential is determined by the characteristic of the electrified particle substance.
When a voltage having the positive polarity is applied to the ejection electrode, the positive-polarity potential of the ink solution is raised. In this case, the electrified toner particles are moved to the front end of the ejection electrode through the ink solution due to the electric field working between the ejection electrode and the counter electrode. The electrified toner particle particles moved up to the front end of the ejection electrode are strongly attracted to the counter electrode side due to the electric field working between the front end of the ejection electrode and the counter electrode. When the Coulomb force working between the electrified toner particles present at the front end of the ejection electrode and the counter electrode greatly exceed the surface tension of the ink solution, agglomerations of the electrified toner particles having a small amount of liquid fly from the front end of the ejection electrode toward the counter electrode and attach to the surface of a recording medium. Thus, by applying a voltage to the ejection electrode, the agglomerations of the electrified particle substance successively fly from the front end of the ejection electrode and printing is executed.
Next, such a kind of conventional electrostatic ink-jet recording apparatus is explained hereunder.
In FIG. 7, the surface of a substrate 54 of an ink-jet recording head 53 is covered with an upper cover 53a and a lower cover 53b, and a slitted ejection opening 57 for holding ink 14 faces toward a counter electrode 56. Moreover, a plurality of ejection electrodes 54a are printed on the surface of the substrate 54 along the ink jet direction in parallel. These electrodes 54a are connected to a not-illustrated voltage driving section so that high-voltage pulses are selectively applied to the ejection electrodes 54a at the time of recording. Furthermore, counter electrode 56 is arranged on the extension line of the ejection electrodes 54a through recording medium 58 to generate an electric field between the counter electrode 56 and the ejection electrodes 54a at the time of recording. In this case, because the ejection electrodes 54a are acicular, an electric field is concentrated on the tip of the ejection electrodes 54a at the time of recording and electric charges are accumulated in the ink 14 nearby the ejection electrodes 54a.
For the toner particles to fly, the toner potential of the ejection electrodes 54a must rise up to a potential high enough for the toner particles to fly. When the toner particles potential exceeds an ejection potential capable of ejecting the toner particles, the toner particles fly toward the counter electrode 56.
However, the toner particle potential fluctuates in the time until reaching the ejectable potential because a characteristic of the ink 14 such as toner concentration fluctuates. This causes a problem that ejection of toner particles having been ejected by applying a voltage to the ejection electrodes 54a for a predetermined time is stopped due to the fluctuation of the ink characteristic.
For example, ejection of toner particles is stopped when the potential of the toner particles does not reach the ejectable potential during a predetermined time for applying the ejecting voltage. However, when a time until reaching the ejectable potential becomes too short in comparison with the predetermined time, a problem occurs that, though toner particles can be ejected for shorter time, the amount of toner particles to be ejected increases because a voltage is excessively applied and thus, the dot diameter increases on a recording medium.